The distribution within primates of nucleic acid sequences related to endogenous retroviruses, their sequence organization and the factors influencing their biological activity was studied. Specifically, (1) to detect and characterize the virus-related sequences in primates and the factors involved in their evolution; (2) to develop probes appropriate for the detection of virus-related sequences in human cells; (3) to characterize the organization of the 50-100 copies of endogenous sequences through restriction mapping and recombinant DNA techniques; (4) to correlate the extent of viral expression with the state of integration of viral sequences, both endogenous and exogenous, through restriction mapping and transfection; (5) to investigate other factors involved in the expression of viral sequences in primates.